1. Field
Embodiments of the present invention generally relate to light emitting diodes (LEDs) on low thermal resistance substrates. In particular, embodiments of the present invention relate to high power density, high fill-factor, micro-channel-cooled ultraviolet (UV) LED lamp head modules that provide high brightness, high irradiance and high energy density.
2. Description of the Related Art
Today's UV LEDs remain relatively inefficient (typically, operating at about 15% efficiency when operated at high current densities). These inefficiencies result in the production of large quantities of waste heat and therefore requiring at least air cooling and often liquid cooling (e.g., heat exchangers and/or chillers) to remove the unwanted waste heat, which is a by-product of the electrical to optical conversion process within the p-n junction of the semiconductor device. If the heat is not removed in a very effective and efficient manner, the LED devices may suffer loss of efficiency, decrease in light output and even catastrophic failure.
Liquid-cooled UV LED lamps (or light engines) are currently being used in a variety of curing applications; however, existing systems have several limitations. For example, while industry literature acknowledges the desirability of high brightness/high irradiance arrays, currently available UV LED lamps provide sub-optimal performance. Existing UV LED lamps generally tend to electrically connect the LEDs within their LED arrays in strings of series-connected LEDs and then parallel these strings together (often with integrated resistors). One drawback to this series-parallel methodology is that the heat sinks usually have to be of a non-electrically conductive nature and/or there needs to be a dielectric layer underneath the LED(s), either of which is traditionally patterned with electrically conductive circuit traces. These traces are expensive and incompatible with thermally efficient ultra-high current operation because of the contact thermal resistance of the layers involved and/or the bulk thermal resistance of the dielectric layer and/or the inherently high electrical resistivity of traces. The heat sinks are also often of expensive ceramic materials such as BeO, SiC, AlN, or alumina. Another disadvantage to the series-parallel LED array model is that a single failure of an LED can lead to the failure of the whole string of seriesed LED(s). This dark area created by a failure in any given chain of LEDs is usually detrimental to the process where the light photo-chemically interacts at the work piece surface.
A specific example of a prior art UV LED array is illustrated in FIGS. 1A and 1B. In this example, which is taken from US Pub. No 2010/0052002 (hereafter “Owen”), an alleged “dense” LED array 100 is depicted for applications purported to require “high optical power density”. The array 100 is constructed by forming micro-reflectors 154 within a substrate 152 and mounting an LED 156 within each micro-reflector 154. The LEDs 56 are electrically connected to a power source (not shown) through a lead line 158 to a wire bond pad on substrate 152. The micro-reflectors 154 each include a reflective layer 162 to reflect light produced by the associated LED 156. Notably, despite being characterized as a “dense” LED array, LED array 100 is in reality a very low fill-factor, low brightness, low heat flux array in that the individual LEDs 156 are spaced quite some distance apart having a center-to-center spacing of about 800 microns. At best, it would appear the LEDs account for approximately between 10 to 20% of the surface area of LED array 100 and certainly less than 50%. Such low fill-factor LED arrays can create an uneven irradiance pattern which can lead to uneven curing and visually perceptible anomalies, such as aliasing and pixelation. Additionally, the micro-reflectors 154 fail to capture and control a substantial amount of light by virtue of their low angular extent. Consequently, array 100 produces a low irradiance beam that rapidly loses irradiance as a function of the distance from the reflector 154. It is to be further noted that even optimally configured reflectors would not make up for the low brightness of LED array 100 as the ultimate projected light beam onto the work piece can never be brighter than the source (in this case LED array 100). This is due to the well-known conservation of brightness theorem. Furthermore, Owen also teaches away from the use of macro-reflectors due to their size and the perceived need to have a reflector associated with each individual LED 156.
The aforementioned limitations aside, the relatively large channel liquid cooling technology employed in prior-art cooling designs is not capable of removing waste heat from the LEDs in a manner that would be effective in keeping junction temperatures adequately low when the current per square millimeter exceeds approximately 1.5 amps.
Oxygen inhibition is the competition between ambient oxygen reacting with the cured material at a comparable rate as the chemical cross-linking induced by the UV light and photoinitiator (PhI) interaction. Higher irradiance is known to create thorough cures more rapidly and higher irradiance is known to at least partially address oxygen inhibition issues. Ultra high irradiance is now thought to perhaps overcome oxygen inhibition issues in certain process configurations perhaps even without a nitrogen cover gas. However, to produce ultra high irradiance to overcome oxygen inhibition, the heat flux removal rate needed to keep junction temperatures adequately low in such a high fill-factor LED array environment operating at extremely high current densities and is simply not attainable with currently employed UV LED array architectures and UV LED array cooling technologies.